1. Field of the Invention
This invention relates to arrangements for reducing or cancelling ambient noise perceived by a listener using an earphone. In this application, the term “earphone” is intended to relate to a device incorporating a loudspeaker disposed externally of the ear of a listener; for example as part of a “pad-on-ear” or “shell-on-ear” enclosure or as part of an assembly, such as a mobile phone, which is held close to the ear.
2. Description of the Related Art
The loudspeaker of the earphone may be coupled to a source of speech or other sounds which are to be distinguished from ambient noise, or the loudspeaker may be provided solely for the reduction of ambient noise, but the invention has special application to earphones used with mobile electronic devices such as personal music players and cellular phones.
At present, some earphones are wired directly to their sound source via short leads and connectors, and some are connected via wireless links, such as the “Bluetooth” format, to a local sound generating device, such as a personal music player or cell-phone. The present invention can be used with both wired and wireless formats.
Existing ambient noise-cancellation systems for earphones are based on one or the other of two entirely different principles, namely the “feedback” method, and the “feedforward” method.
The feedback method is based upon the use, inside the cavity that is formed between the ear and the inside of an earphone shell, of a miniature microphone placed directly in front of the earphone loudspeaker. Signals derived from the microphone are coupled back to the loudspeaker via a negative feedback loop (an inverting amplifier), such that it forms a simple servo system in which the loudspeaker is constantly attempting to create a null sound pressure level at the microphone. Although this principle is simple, its implementation presents practical problems which limit the upper frequency of operation, to about 1 kHz or below. Furthermore, effective passive acoustic attenuation must be provided to prevent the ingress of ambient noise above this 1 kHz limit, and this is done by providing an ear-enclosing circumaural seal, designed to block these frequencies. A recent attempt to improve the performance of feedback systems is described in US 2005/0249355 A1.
Still further, if music or speech is to be fed to the user's earphone, then provision must be made to avoid these wanted signals being cancelled out by the feedback system, and this process can introduce undesirable spectral troughs and peaks into the acoustic characteristic of the earphone. Moreover, a feedback system of this type requires that the operating cavity is substantially isolated from the ambient and, although “pad-on-ear” feedback devices were proposed some twenty years ago, it is believed that no earphones of this type are yet commercially available. Feedback systems are susceptible to go into “howl around” oscillation at switch on or when operating conditions change.
Arrangements in accordance with the present invention thus utilize exclusively the feedforward principle, which is shown in basic form in FIG. 1.
In feedforward operation, a microphone A is placed on the exterior of an earphone shell B in order to detect the ambient noise signal. The signal detected by the microphone A is inverted at C and added to the drive signal applied to a loudspeaker D, thus creating the “cancellation signal”. The intention is that destructive wave cancellation occurs between the cancellation signal and the incoming ambient acoustic noise signal, adjacent to the earphone loudspeaker outlet port within the cavity between the earphone shell B and the outer ear E of a listener. For this to occur, the cancellation signal must have a magnitude which is substantially equal to that of the incoming noise signal, and it must be of opposite polarity (that is, inverted, or 180° shifted in phase with respect to the noise signal).
The earphone shell B typically carries a foam pad F, or a similar device, in order to provide a comfortable fit to the outer ear E of the listener, and/or to assist in reducing the ambient noise reaching the listener's ear.
Feedforward ambient noise cancellation is, in principle, simple to implement. A basic working system for use with ordinary earphones can be assembled at very low cost using a simple electret microphone capsule and a pair of operational amplifiers to amplify and invert its analogue signal, prior to mixing with the earphone audio drive signal. This is done via an adjustable gain device, such as a potentiometer, in order to adjust the magnitude of the cancellation signal to equal that of the ambient noise. Some measure of noise cancellation can be achieved with this method, but it is far from perfect. Nevertheless, the feedforward principle forms the basis of numerous earphones which are now commercially available. However, even when the cancellation signal is optimally adjusted and balanced, a considerable residual noise signal still remains, and so it is common to observe that most commercially available systems are only claimed to operate below about 1 kHz, thus providing only a slightly greater bandwidth than that of the feedback method. Bearing in mind that the voice spectrum extends to 3.4 kHz, any associated noise-cancellation system demands a bandwidth well in excess of the capabilities of currently available systems in order, for example, to significantly improve the intelligibility of dialogue via a telecommunications link.